The present invention relates to an offset printing machine or press, and more particularly to an offset printing machine provided with a clamp adjusting member for mounting a printing plate onto a plate cylinder. The invention relates to a printing plate wherein lines for detecting a displacement or offset of an image position relative to a regular or an exact position are formed. It is also concerned with an image position reading-out method for detecting a displacement or an offset of an image position of a printing plate.
In general, in the printing work, it is necessary to print the images in parallel to the printing paper. It is therefore necessary to position the images of the printing plate relative to the plate cylinder. However, it would be very difficult to position the images parallel to a clamping portion of the printing plate. The images would often be slanted to the clamp portion of the printing plate. Also, a distance from the clamp portion would be changed in every print. There would be no image in a center of the printing plate. There would be a fear that a printed article would be obtained with the images being twisted to the clamp portion of the printing plate during the printing operation. Accordingly, in the case where after a few pieces of prints or printed article have been obtained in the initial stage of the printing operation, the images are twisted, the printing plate must be once peeled off and again clamped in place. Such operation will be referred to as "reclamping".
Accordingly, various methods have been proposed to dispense with the reclamping operation.
There is a first method for laying an original on an original table of a printing machine, which utilizes a pin system during the printing operation, and prints the images in parallel to or perpendicular to a marginal portion of the printing plate.
There is a second method for cutting the clamp portion of the printing plate for every printing in parallel to the images after the printing operation has been effected.
A third method is that a twist of the slanted image is adjusted by moving a table by a knob (Japanese Utility Model Publication No. 58-4670 and Japanese Utility Model Laid-Open Publication No. 60-127929).
A fourth method is that a twist adjustment mechanism is provided within a plate cylinder shaft supported on the plate cylinder, a bracket is moved along the plate cylinder shaft, and a position of a clamp claw is adjusted (see Japanese Patent Laid-Open Application No. 61-125847). In this method, a rotation of the plate cylinder shaft is adjusted step by step through the engagement between a claw and a ratchet wheel. In this case, a register mark having a special shape is printed together with the image; the register mark is read out; a displacement in distance of the image at the clamp portion (vertical direction), a displacement of the image relative to the center of the print and an amount of slant of the image are calculated; and finally, the position of the print relative to the plate cylinder is corrected on the basis of these data.
There is a fifth method in which a displacement of the position of the image is detected by effecting a register mark on the print and the displacement is corrected as shown in Japanese patent Laid-Open Publication No. 59-123665.
Although the "reclamping" operation of peeling the print once mounted on the plate cylinder and again clamping the print might be dispensed with in accordance with the first through third methods, any of these methods suffers from a difficulty such that the operation needs a long period of time.
Turning to the fourth and fifth methods, since special marks such as a register mark B in the fourth method (Japanese Patent Laid-Open Publication No. 61-125847) and horizontal and oblique register marks in the fifth method (Japanese Patent Laid-Open Publication No. 59-123665) are used, an error due to a shape of the mark shape itself would become an actual error in measuring a displacement amount, i.e., a movement amount. In addition, it is necessary to provide a special jig and an special apparatus for producing a special register mark.
Also, in the fourth method (Japanese Patent Laid-Open Publication No. 61-125847), since only distances I.sub.1 and I.sub.2 from a predetermined reference position x--x to the register marks B and C are read out, if lines of the register marks B and C would be thin during the printing operation, this would be an error of the displacement amount, i.e., movement amount. In addition, since the reading-out method is based upon the distances I.sub.1 and I.sub.2 from the reference position, if the setting of the print would be improper, this would be an error. Moreover, in the fourth method, since the rotation of the plate cylinder is stepwise adjusted, if would be impossible to finely adjust the rotation.